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Interpretation of the Central Metamorphic terrane metabasites as the lower-plate of a Devonian intraoceanic subduction zone is a basic tenet of paleotectonic models for the eastern Klamath province. Specifically, such models hold that the (1) east-dipping Central Metamorphic terrane represents a subducted piece of oceanic lithosphere accreted to the overlying Trinity subterrane; (2) Trinity and Redding subterranes represent mantle and magmatic-arc crust of the overriding lithospheric plate; and (3) east-dipping Trinity thrust, separating the Central Metamorphic terrane and Trinity subterrane, represents the paleo-plate boundary. The subduction-zone model predicts that mid-ocean ridge basalt (MORB)-type oceanic basalts formed the protolith of Central Metamorphic terrane metabasites. We present new major (X-ray fluorescence spectrometry) and trace (inductively coupled plasma-mass spectrometry) element data for twenty Central Metamorphic terrane metabasites; these geochemical data demonstrate that (1) seafloor hydrothermal alteration had little or no effect on protolith compositions; (2) original igneous fractionation trends are preserved by immobile, incompatible trace elements; and (3) immobile trace elements preserve a distinct normal-MORB (N-MORB)-type composition. Fluid-mobile elements show varying degrees of enrichment relative to MORB, suggesting post-magmatic metasomatism. Based on these results, we conclude that the Central Metamorphic terrane represents oceanic lithosphere accreted to the upper-plate of an intraoceanic subduction-zone. Two new Early Permian 40Ar/39Ar ages of 274 ± 2 Ma are reported for Central Metamorphic terrane amphibolite near the Trinity fault are considerably younger than the previously reported Devonian age. These ages reflect Early Permian uplift and cooling of the Central Metamorphic terrane during a proposed episode of supra-subduction extension that reactivated the Devonian subduction-margin (Trinity thrust).

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